1. Field of the Invention
The present invention relates generally to an image display apparatus and, more particularly, is directed to an image display apparatus for displaying an image by the raster-scanning of laser beams.
2. Description of the Prior Art
In order to better understand the present invention, a previously proposed projection-type video display apparatus (see Japanese Utility Model Laid-Open Gazette No. 56-152456) is first explained with reference to FIG. 1.
Referring to FIG. 1, there are provided laser light sources 1a and 1B such as a semiconductor laser, gas laser and the like. A red laser light beam from the laser light source 1a is introduced to an optical modulator 2a. A green laser light beam and a blue laser light beam from the laser light source 1b are introduced into a dichroic mirror 3a, in which they are separated to provide a green laser light beam and a blue laser light beam. The green laser light beam is introduced to an optical modulator 2b, whereas the blue laser light beam is introduced through a reflection prism 4a to an optical modulator 2c. Modulation signals corresponding to three primary color signals, provided as video signals of an image to be displayed, are supplied to the optical modulators 2a, 2b and 2c, respectively. The optical modulators 2a, 2b and 2c are adapted to modulate intensities of the red, green and blue laser light beams on the basis of the modulation signals. The laser light beams derived from the optical modulators 2a, 2b and 2c are respectively supplied to lenses 5a, 5b and 5c for adjusting the diameters of the beams. The blue laser light beam from the lens 5c is introduced through a reflection prism 4b to a dichroic mirror 3b, and the green laser light beam from the lens 5b is introduced to other mirror face of the dichroic mirror 3b, whereby the blue and green laser light beams are mixed. The laser light beam thus mixed is introduced to a dichroic mirror 3c, and the red laser light beam from the lens 5a is introduced to the other mirror face of the dichroic mirror 3c, thereby obtaining a mixed laser light beam of three primary color beams. This mixed laser light beam is made incident on a reflection portion 11 of a polygon mirror 10.
In the polygon mirror 10, the reflection portion 11 is formed of flat mirrors arranged with an equal spacing in an annular fashion. This annular reflection portion 11 is rotated at high speed by some suitable drive means such as a motor and the like. The flat mirrors forming the reflection portion 11 form, for example, a regular icosipentahedron and deflect the laser light beam incident on the respective flat mirrors. FIGS. 2A and 2B illustrate the deflected states of the laser light beam.
As, for example, shown in FIG. 2A, when a laser beam lin becomes incident on an end portion of a flat mirror 11.sub.1 of the reflection portion 11 by the rotation, a laser beam lout is reflected toward the downward portion as shown in FIG. 2A. With the rotation of the reflection portion 11, the incident angle of the laser beam lin to the flat mirror 11.sub.1 gradually changes, and thereby the outgoing direction of the reflected laser beam (out changes upwardly. As shown in FIG. 2B, when the reflection portion 11 is rotated by an angle .theta..sub.1 to cause the laser beam lin to become incident on the other end portion of the flat mirror 11.sub.1, a laser beam lout' is reflected toward the upper portion of FIG. 2B. In that event, the angle .theta.2 between the laser beam lout and the laser beam lout' becomes the deflection angle by the flat mirror 11.sub.1. Other flat mirrors of the reflection portion 11 deflect the laser beams by similar deflection angles so that, when the reflection portion 11 is formed of 25 flat mirrors, the laser beam is deflected 25 times per rotation of the reflection portion 11.
Referring back to FIG. 1, the laser beam reflected on the polygon mirror 10 is introduced through a projection lens 6 to a galvano mirror 7. This galvano mirror 7 is rotated by a drive source 7a. When the galvano mirror 7 is rotated at a predetermined interval by the drive source 7a, the laser beam from the polygon mirror 10 is deflected at a predetermined deflection angle during a predetermined interval. In this case, the deflection direction of the polygon mirror 10 and the deflection direction of the galvano mirror 7 are selected to be perpendicular to each other. The deflection by the polygon mirror 10 corresponds to the horizontal position on a screen 9 in the television receiver, whereas the deflection by the galvano mirror 7 corresponds to the vertical position on the screen 9 in the television receiver.
The laser beam reflected by the galvano mirror 7 is reflected by a reflection mirror 8 and becomes incident on the rear surface of the screen 9. Upon use, the viewer can see an image, formed by the laser beams, from the front surface of the screen 9.
Horizontal and vertical scanning periods of the video signal for forming the modulation signals supplied to the optical modulators 2a, 2b and 2c are synchronized with the deflection period by the polygon mirror 10 and the deflection period by the galvano mirror 7, whereby the image based on the video signal is displayed on the screen 9 by the raster-scanning of the laser beams. Thus, an image of one field is displayed on the screen 9 during one field period of the video signal, and hence, the display apparatus shown in FIG. 1 is operated as a projection type video display apparatus.
Next consider how the display apparatus thus constructed is operated in practice. For example, when a video signal having 1125 horizontal scanning lines such as a high definition television signal is displayed, the polygon mirror having 25 flat mirrors has to be rotated at 81000 r.p.m., which requires a special drive motor and a bearing for such very high speed rotation. If the polygon mirror is rotated at such high speed, then the flat mirrors of the polygon mirror also move at high speed, which is undesirable from a safety standpoint.
In order to solve the above-noted problems, one could attempt to decrease the rotation speed of the polygon mirror by increasing the number of the flat mirrors formed on the reflection portion of the polygon mirror. This proposal is not practical for several reasons:
If the diameter of the rotating portion of the polygon mirror is increased, then a centrifugal force applied to the reflection portion of the polygon mirror will increase, thereby causing an elastic strain on the mirror surface. Thus, the raster-scanning of laser beams is disturbed, and a motor having a large torque is needed, which is not practical.
Further, if the number of the flat mirrors is increased without changing the diameter of the rotating portion of the polygon mirror, then the area of one flat mirror will decrease, reducing the deflection angle from the standpoint of a beam spot of a laser beam, which is also not practical.
The above-noted problem will be explained more fully. For example, the flat mirror formed on each plane of the polygon mirror having a pentacontane reflection surface and a diameter of 40 cm has a width of 2.5 mm. When a laser light beam having a diameter of 1 mm becomes incident on the above-mentioned polygon mirror, if the laser beam becomes incident on adjacent planes at the boundary portion of the flat mirrors, then a so-called eclipse will occur. Consequently, part of the respective end portions of each plane 1/2 mm at each end are unusable, i.e. 40% of each plane cannot be used. Thus, only 60% of the plane can be effectively utilized, and the deflection angle becomes very small, which is not practical.
Furthermore, if the size of the beam spot of the laser beam is reduced, then the resolution of an image displayed will be degraded. Therefore, it is not possible to reduce the unusable area by reducing the beam spot of the laser beam.
In order to solve the above-mentioned problem, the following proposal is made, and this proposal will be explained with reference to FIG. 3.
As shown in FIG. 3, a laser light beam from a laser light source 1 is made incident on the reflection portion 11 of the polygon mirror 10 in the direction perpendicular to the rotation axis of the polygon mirror 10 and at an angle relative to the reflection portion 11 of the polygon mirror 10. The laser beam reflected by the reflection portion 11 is reflected by a fixed flat mirror 13 so as to become incident on the reflection portion 11 of the polygon mirror 10 one more time so that this reflected laser beam is introduced to a light path substantially behind the polygon mirror 10. Thus, when the reflection portion 11 of the polygon mirror 10 is rotated by, for example, .DELTA..omega., if the fixed flat mirror 13 is not provided and the laser beam is reflected one time, then the deflection angle of the reflected laser beam will become .alpha.1. If the fixed flat mirror 13 is provided and the laser beam is reflected twice on the reflecting portion 11, then the deflection angle will be doubled to .alpha.2. As described above, if the fixed flat mirror 13 is provided, then the deflection angle of each plane of the polygon mirror 10 will be so extended. Thus, the number of flat mirrors forming the reflecting portion 11 can be increased without varying the radius of the polygon mirror 10.
In the example of FIG. 3, however, as the positional relationship between the primary incident point a and the secondary incident point b on each flat mirror or plane of the reflecting portion 11 is changed with the rotation of the polygon mirror 10, it happens that the secondary incident point b is on the adjacent flat mirror rather than on the same mirror as the primary incident point a. FIG. 4 shows how the loci of the incident points a and b are changed with time. As shown in FIG. 4, when the primary incident point a approaches an end portion of the predetermined plane 11.sub.1 of the reflecting portion 11, the secondary incident point b changes to the adjacent plane 11.sub.2 as shown by an arrow to thereby disturb the deflection angle.
As shown in FIG. 5, in another prior art apparatus the incident angle of the laser light beam from the laser light source 1 is inclined from the perpendicular direction of each plane of the reflecting portion 11 by a very small inclination angle and the primary and secondary incident angles are provided close to each other, thereby preventing the reflected light beam from the fixed flat mirror 13 from being moved toward the adjacent plane. However, in this apparatus, a reflected light beam deflected can not reach the interval S which is a rear side of the flat mirror 13, and the laser light beam is interrupted in this interval S by the flat mirror 13.